The advent of microdevice technology for biochemical and chemical analysis has begun to revolutionize the world of science. While the microchip revolution is rooted in ultra fast separations, recent forays seek to move laborious and time-intensive steps for sample chemical and biological identification to microchips. Success with these developments will supplant the use of multiple instruments in the laboratory with the cost-effectiveness and rapid response of using a single miniaturized rapid analysis system. Many of these emerging total analysis systems (μ-TAS) or “lab-on-a-chip” sensor systems can simply be interfaced with a computer for automated, user-friendly applications.
A large breadth of biological and chemical analyses will be possible with microdevices having multifunction capabilities. The key to creating multifunctional devices with turn-key operation capability will be the integration of processes for total analysis. For example, for genomic analysis, the totally integrated analysis would require that steps such as cell sorting, cell lysis, DNA extraction, DNA quantitation, and DNA amplification (via PCR) be carried out on-chip prior to the analysis step (e.g., electrophoresis or microarray) on the same microdevice.
One of the important issues for proper function of a μ-TAS is the ability to detect a wide variety of chemical and biological analytes, such as ions, drugs, DNA and proteins. A variety of sensors have been developed to indicate the presence of specific species, including quartz crystal microbalances, surface plasmon resonance sensors and fluorescence-based sensors. However, there is a significant need for simple, inexpensive sensors that exhibit high sensitivity, rapid response, and smaller size to facilitate integration into μ-TAS that might be portable or hand-held.
One approach to creating highly specific and sensitive detectors is to couple surface adsorption and mechanical deformation of microfabricated cantilevers. If the surface is functionalized to be chemically-selective, then bending of the cantilevers is a clear indicator of the presence of target molecules. Current approaches utilize silicon cantilevers which are microfabricated using well-established sacrificial surface micromachining techniques. The surface of the cantilever is then functionalized by applying a thin coating that is used to control the specific adsorption of molecules/analytes: for example, gold coatings are used with well-established gold-thiol chemistry to make sensors that are chemically-selective for DNA (Marie et al., Ultramicroscopy 91:29-36, 2002) or prostate specific antigens (PSA) (Guanghua et al., Nature Biotechnology 19, 856-860, 2001). This approach has the distinct advantage of the ability to perform label-less detection, i.e., the analytes do not need to be labeled with a fluorescent tag. By using microfabricated arrays of many cantilevers, the potential exists to combine multiple surface functionalizations to create multiplexed assays that can be scanned in an automated fashion to indicate the simultaneous presence of multiple analytes.
U.S. Pat. No. 6,289,717 to Thundat et al. discloses a sensor for detecting specific binding reactions. The sensor provides a cantilever with one of its surfaces coated with specific binding partners, while the other surface is covered with a different, possibly inert, material. As long as the amount of adsorption is different on the opposing surfaces, or there are different interactions of monitored molecules on opposing surfaces, there will be a differential stress. The specific binding interaction is manifested as changes in the differential surface stress of the cantilever surface. If a specific interaction does not take place, there will not be any change in surface stress when compared to a reference microcantilever. These changes in differential surface stress manifest themselves as changes in cantilever deflection which can be measured with a sub-angstrom sensitivity.
U.S. Published Patent Application No. 2006/0075803 to Boisen et al. improves the cantilever of Thundat et al. by using polymer-based cantilever in an array for high throughput screening, and improved stability and sensitivity, but still requires some form of optical interrogation.
Although such chemo-mechanical sensor concepts have proven their utility, current devices are limited by to several disadvantages. First, the use of silicon (or other inorganic materials) limits both sensitivity (due to the large stiffness of such materials) and robustness (due to the small failure strains and brittle nature of such materials) of such devices. The stiffness of these materials is particularly problematic; adsorption typically induces cantilever deflections on the order of 10-100 nm (Lavrik et al., Review of Scientific Instruments 75:2229-2253, 2004; Lang et al., Materials Today, April, 30-36, 2005), which requires sophisticated laser-based detection hardware. Second, prior art cantilevers are stand alone detectors that are not incorporated into an integrated μ-TAS, possibly due to the required sophisticated detection system that cannot be integrated directly into a μ-TAS.
Consequently, there remains a critical need to develop new approaches that: (i) exhibit much larger deformations upon adsorption, so that more simplistic and cost-effective transduction mechanisms (such as capacitance, resistance, optical interferometry, simple visual (color change) detection) can be utilized, and (ii) incorporate materials and microfabrication processes that enable transduction to be directly integrated into the microchip itself.